This invention relates generally to high temperature, magnetically confined fusion plasmas and in particular to the study and measurement of the behavior of energetic alpha particles heated to sustain the high temperatures required in a fusion plasma.
One approach currently under evaluation in the development of nuclear fusion as a long term energy source involves the magnetic confinement of an energetic plasma in the form of a toroid or "doughnut". This has given rise to the tokamak fusion reactor design which is currently under intensive study by research groups in a number of countries. By means of a circular arrangement of powerful magnets, a toroidal magnetic field is formed wherein is confined the energetic plasma comprised primarily of protons, deuterons, tritons and electrons.
Significant quantities of energetic alpha particles produced by deuterium-tritium fusion reactions are expected to be produced in the next generation of magnetic confinement fusion devices such as the Tokamak Fusion Test Reactor (TFTR) and the Joint Experimental Tokamak (JET). It is also expected that substantial heating of the plasma by the deuterium-tritium fusion-produced alpha particles would occur if they slow down classically. How well the energetic alpha particles are confined within the plasma will be one of the most significant questions to be answered.
Given the large costs and lead times associated with reactor-sized experiments, it is desirable to learn as much as possible about energetic alpha particle behavior within the plasma in general, and, in particular, to determine as early as possible whether the fusion-product alpha particles slow down in a classical manner through binary coulomb collisions or whether they instead are subject to anomalous processes prior to thermalization. These anomalous processes could, for example, lead to loss of the fast alphas from the central portion of the plasma resulting in accelerated erosion of the reactor first wall, change the rate at which the fast alpha particles slow down and heat the ions and electrons in the plasma so as to increase the difficulty of sustaining the fusion reaction, or lead to a reduction in ignition requirements due to an as yet unknown in ion heating mechanism by the alpha particles. In general, such instabilities affecting alpha particle transport and plasma heating have the potential of changing the ignition and confinement properties of a fusion plasma. Because several near term fusion experimental reactors are expected to produce significant numbers of alpha particles from fusion reactions, a unique opportunity will be available to study the physics of alpha particle heating and confinement, an understanding of which is critical to future fusion reactor design and development.
The TFTR and JET experiments will use deuterium and tritium with large power auxiliary heating. These experiments are expected to have energy multiplication factors of .apprxeq.1-2, with the result that the alpha particle heating will be .apprxeq.15 to 30% of the total heating, with alpha heating power densities of 0.6 watts/cm.sup.3 or less. Thus, alpha particles will play only a minor role in the power balance of these experimental devices. Temperature excursions due to alpha particle heating will be only .apprxeq.10-20% effects and will not provide a definitive measure of alpha particle confinement characteristics, especially since it is expected to take a fairly large number of discharges to change from a deuterium-deuterium plasma to a deuterium-tritium plasma to make the comparison. Systematic errors, measurement uncertainties, and the lack of shot-to-shot reproducibility will make it difficult to draw definitive conclusions from the 10-20% measured temperature differences.
Also produced in the deuterium-tritium fusion reaction, besides the 3.5 MeV alpha particle, is a 14 MeV neutron. The escape of these energetic neutrons from the confined plasma and the resulting collisons with various reactor structures and components will result in the "activation" of these materials wherein the atoms of these materials are converted into radioisotopes. The neutron activation from only a relatively modest number of deuterium-tritium discharges will be large enough to require remote handling maintenance, thereby substantially increasing the complexity and expense of maintenance operations in these experiments, and rendering impossible the repair of a number of types of machine failures in these experimental reactors. In addition, the use of tritium in these experiments will result in various radiological hazards which can only be dealt with at great expense and inconvenience.
Accordingly, the present invention is intended to overcome the aforementioned problems of the prior art by providing a means for simulating energetic alpha particle behavior in a magnetically confined plasma without the neutron activation problems heretofore encountered. The present invention makes use of energetic .sup.3 He.sup.++ ions to simulate high energy alpha particle heating without neutron producing fusion reactions.